MECHANICAL SEPARATION 315 by the greater rim speed of the distributor, and the liquid is projected through the gas space in the form of a fine, high-velocity spray, which, upon impact with the shell, is broken into a mist or fog. This alternating humidifying and scrubbing effect to which the gas is subjected in each chamber provides the intimacy of contact between gas and liquid necessary for the removal of finely divided insoluble matter or for the recovery of soluble constituents existent in extreme dilution, and also provides for the almost universal application of the apparatus as a scrubber as well as a reaction or absorption tower. The vigorous liquid agitation eliminates the possibility of internal stoppages, as has been demonstrated on scrubbers operating for several years using liquids carrying suspended, insoluble, reactive material. Any liquid entrainment is removed from the gas by means of the deflectors or baffles in the dome, and the collected liquid is returned through the drain to the top chamber. For primary washing, the Feld scrubber is ordinarily constructed with seven chambers or sections; the lower three being the washing chambers, the fourth one being a separating chamber and the upper three being the cooling chambers. The washing is accomplished mostly in the lower sections, whereas the upper sections perform primarily the function of cooling and dehumidifying the gas, all of the functions being subject to variation at will, depending upon the amount of gas passed through the scrubber and upon the volume and temperature of the scrubbing liquid used. A Feld gas scrubber of standard design 5-ft. diameter, is estimated to be capable of cleaning 200,000 to 250,000 cu. ft. of gas per hour and is driven by a 10-hp. motor. The volumes of flow of both the gas and the liquor are at all times under control and can be varied and relatively proportioned to meet operating conditions within the rated capacity of the scrubber. It may readily be shown by experiment that when drops of water are broken up (as in falling raindrops, or by atomizing water by means of spray nozzles), the particles become positively charged with electricity. The positively charged particles condense more readily on negatively charged particles or on negative ions than they do on positively charged particles, and, also, liquid drops take up negatively charged particles or ions more readily than positive ones. When dust is blown about by the wind, it becomes charged electrically, and from experiments made by Rudge, the following generalizations were deduced: 1. Non-metallic elements give positively charged clouds when the finely divided solid material is blown into a cloud by a current of air. 2. In similar manner, metallic elements give negatively charged clouds. 3. Solid acid-forming oxides give 'positively charged clouds. 4. Basic oxides give negatively charged clouds. In the treatment of material containing or constituting salts, the charge apparently depends on the relative strength of adsorption of positive and negative ions. Finely divided metallic sulphides generally acquire a positive charge, but the metallic oxides, sulphates, and sulphites acquire negative charges. In general, it has been found that highly dispersed substances, suspended either in gases or liquids, carry electrical charges, and this is particularly true of colloidal and ionized substances. It may be stated that, as a rule, basic colloids acquire negative charges, whereas acid colloids acquire positive charges. The forces effective in the operation of a Feld scrubber then are: The spray or mist particles produced in the scrubber are positively charged, whereas the particles of fume contained in the smelter gases are negatively charged, and, all the physical conditions being such that these oppositely charged particles are brought into most intimate contact, the result is that these particles are mutually attracted and the fume particles are wetted and submerged within the liquid and thus removed from the gas stream.